Capacitor and electrolyte

ABSTRACT

An aluminum electrolytic capacitor having dimethylammonium acid salts in the electrolyte solution.

United States Patent [1 1 Dunkl et al.

[451 Sept. 30, 1975 CAPACITOR AND ELECTROLYTE Inventors: Franz S. Dunkl; Sidne D. Ross,

both of Williamstown; Manuel Finkelstein, North Adams, all of Mass.

Assigncc: Sprague Electric Company, North Adams. Mass.

Filed: Nov. 8, 1974 Appl. No: 522,361

[52] US. Cl. 317/230; 252/622; 260/50l.l [51] Int. Cl. HOlG 9/02 [58] Field of Search 317/230; 252/622.

Primary E.\'unzine"J0hn Kominski Attorney, Agent, or FirmConn0lly and Hulz [57] ABSTRACT An aluminum electrolytic capacitor having dimethylammonium acid salts in the electrolyte solution.

4 Claims, 2 Drawing Figures U.S. Patent Sept. 30,1975 3,909,682

CAPACITOR AND ELECTROLYTE BACKGROUND OF THE INVENTION This invention concerns electrolytic capacitors which include electrodes contacted by a specific type of electrolyte, and more particularly to aluminum electrodes contacted by fully reacted acid salt electrolytes.

Electrolytes for capacitorshaving aluminum electrodes were for many years composed of aqueous solutions of acids. Another prior art electrolyte employed in aluminum electrolytic capacitors has involved the reaction of boric, salicylic or other acid with an amide such as dimethylformamide. This has provided a suitable working electrolyte for many aluminum electrolytic capacitors. However, this electrolyte is limited to low voltage use, because capacitors containing this electrolyte are not stable at high voltages when used over a period of time.

It is an object of the present invention to overcome the foregoing disadvantages in aluminum electrolytic capacitors by employing reacted acidic compounds with substituted amides suitable for use over broad temperature ranges for long periods of time without adverse or detrimental changes in electrical properties.

SUMMARY OF THE INVENTION This invention provides the preparation of an inexpensive electrolyte containing dimethylammonium acid salt for aluminum electrolytic capacitors by the reaction of acidic compounds with the substituted amide to form the dimethylammonium acid salts.

This electrolyte is a reacted mixture of dimethylformamide or dimethylacetamide with an acid, such as salicylic acid, boric acid, and picric acid. The reaction mixture of the dimethylformamide and an acid selected from the particular group of this invention has a low repicric acid sistivity and remains stable indefinitely. Accordingly, a capacitor containing this reaction mixture in the electrolyte system is relatively unchanged by aging of the unit during extended operation.

The present invention involves both the novel electrolyte system containing the reaction mixture and the methods of preparing this improved capacitor electrolyte.

BRIEF DESCRIPTION OFTHE DRAWING FIG. 1 illustrates a capacitor section partially unwound having an anode 11 and a cathode 12. The anode 11 consists of an aluminum foil and cathode 12 consists of a metal foil. Porous spacers 13 and 14 are of a material inert to the electrochemical reactions and are impregnated with an electrolyte according to this invention. Tabs 15 and 16 provide means for connection to the anode l1 and cathode 12, respectively.

The capacitor section may be housed in any conventional capacitor container.

FIG. 2 is a graph of the curve showning the conductivity of a representative electrolyte according to this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Boric, salicylic and other acids with dimethylformamide, hereinafter referred to as DMF, have proven to be useful in capacitor electrolyte systems. With the acid salts of this'invention a high voltage rating stable capacitor is achieved with low resistivity. A group of acids reacted with a substituted amide provide an electrolyte having a resistivity of from about 100 to about 600 ohm-cm. The group of acids includes salicylic acid, boric acid, phosphotungstic acid, and combinations of boric acid with pyrocatechol or naphthalenediol.

An acid heated in DMF at 125C is converted to the dimethylammonium salt in excellent yield. Thus when 0.05 mole of picric acid in 100 ml. DMF is heated 24 hours at 125-130C an 84% yield of dimethylammonium picrate, m.p. 159-162C, can be isolated. This reaction does not involve a prior hydrolysis of DMF to give formic acid and dimethylamine. No formic acid is formed, and if the effluent gasses from the reaction are passed into a solution containing phosphomolybdic acid and palladium chloride, the deep blue coloration indicating the presence of carbon monoxide is observed. The carbon monoxide has also been collected in a gas burette and identified by VPC.

The following examples are illustrative, not limitative of the present invention, showing the preparation of the reaction mixture. The equation with the structural formulae graphically represents the reaction course.

EXAMPLE I picrate ion dimethylammonium DMF CH;, No

CH No A solution of 0.1 mole of -p-toluenesulphonic acid monohydrate in 200 ml. of DMF has a room temperature resistivity of 253 ohm-cm. and an apparent pH of 2.1. After 4 hours at C the room temperature resistivity decreases to 137 ohm-cm and the apparent pH increases to 5.5. Removal of the DMF in vacuo and crystallization of the crude product from acetone-ether yielded 19 g. (87.6%) of dimethylammonium p-toluenesulphonate, m.p. 7679.

EXAMPLE 2.

Anal. Calcd. for C l-l B NO N, 5.30%. Found: N, 5.74%.

acid and the, mixture is reacted with heating to form a solution of dimethylammonium acid salt in DMF. The following observed results are illustrative:

EXAMPLE 3 The reaction was observed with a solution of salicylic 5 Ohm PH acid l 38 g.; 1 mole) and boric acid (30.9 g.; 0.5 mole) b f reacting 206 L7 in DMF (750 ml.), heated overnight at 125C, yielded, after 2 at I58 after removal of the DMF in vacuo and crystallization 6 i C 0 25 from acetone-methanol-ether, 124 g. (83.5%) of dime- 10 Ohm cm. 1090 592 234 163 91.5 74.0 thylammonium borodisalicylate; m.p. 193l95. a

FIG. 2 shows how the resistivity of the electrolyte of EXAMPLE 4 this invention varies with temperature. Similar reactions were observed in dimethylac- L test 0f.l2 i' o at 1255C, Averages: etamide. When dry picric acid (23 g.; 0.1 mole) in freshly distilled dimethylacetamide was heated 24 F hours at 125C, two products were isolated. These were 0 hr 315 7 9 3 0 dimethylammonium picrate, 11.6 g. (42.5%) and the 500 311:6 69 1:1 complex of picric acid and dimethylacetamide, I000 hrs. 307.8 72 1.9 11.5 g. (36.3%). This complex, which melts at 62-64, 75 has been reported previously (V.F. Chesnokov and l.M. Bokhovkin, .I.Gen.Chem.,lJSSR, 3 9, 9 15 1969). EXAMPLE'7 It was prepared 1ndependently 1n quantitatlve y1eld by dissolving picric acid in an equal weight of dimethylac- Borodisalicylate etamlde, cooling and adding ethen 5.3 g. salicylic acid and-l .22 g. boric acid are mixed similar 111 Complexes have also been P P in in 100 g. DMF and the mixture reacted with heating to this way: from picric acid and dimethylformam1de form a Solution of dimethylammonium acid Salt in -P' from N'methylpmpionamide and P DMF. The following observed results are illustrative: ric acid (m.p. 48-49; and, from dimethylacetamide and 2,4-dinitrophenol (m.p. 62-64). Ohm cm, pH

before reacting at 25C 238 1.9 5 after 2 hrs. Bl l25C l 6,3

When a solution of 1:1 dimethylacetamide-picric acid complex (5.0 g.) in n-butylalcohol (8 ml.) was refluxed 48 hours the products obtained were dimethylammonium picrate, m.p. l58l61, 3.4 g. 91.9%)

Life test of 5 units, V at 125C, Averages:

and n-butylacetate, 1.16 g. (63.3%). In this case salt FF RC M formation results in the formation of ketene, rather 40 0hr 31 43 56 30 than carbon monox1de, and the reaction course 1s as (5)00 hrs. 311.4 77 2.2 Shown below 1 00 hrs. I 302.6 97 1.63

NO. 11., No.

- CH =C=0 CH -,CH CH-,CH OH CH;.CH CH CH,OCCH

The reaction mixtures of this invention have been EXAMPLE 8 found to provide capacitor electrolytes which have a low loss of conductivity at low temperature, stability at prolonged exposure to high temperature, and low leakage current of the capacitors made with it.

The following examples describe electrolytes according to this invention containing solutions of dimethylammonium acid salts in DMF. The observed characteristics illustrate the advantages.

Dimethylammonium borodisalicylate 6.58 g. of dimethylammonium borodisalicylate in a DMF solution at 25C has a resistivity of 136 ohm cm and a pH of 9.1. The following is illustrative:

Life test of 11 units, 40V at'125C, Averages:

F EXAMPLE 6 0 h phosphowngstlc Acld 500 hl's. 302.3 6' l 1000 hrs. 308.2 59 l.3

100 g. DMF has added to it 18 g. phosphotungstic I 7 EXAMPLE 12 EXAMPLE An electrolyte according to this invention was pre Borodisalicylate pared with mixingpyrocatechol in DMF and heating 5 g. of salicylic acid and 2.5 g. of boric acid are mixed 5 wherein the following formulation was s prepared:

in 100 ml. of DMF and the mixture reacted with heating to provide a product showing the following illustrative results. DMF 79 Glycol 5 g. 10 Boric Acid 10 g. O cm pH Catechol 0.26 g. (0.00238 moles) Tri-Butylamine 5.5 g. (0.0298 moles) before reacting at 25C 229 1.9 after 2 hrs. at l25C 149 6.5 after 16 hrs. at 125C 149 65 It was determined that, as mltially prepared, this electrolyte had a resistivity of 603 ohms-cm. at 25C and had the following voltage capabilities:

Formation'of aluminum foil in this electrolyte shows the following:

v .25C Alf '1 b '1 d: 440 Max. formation voltage at: max 2 2,221: i 406 20 v max. 125C boiled foil: 455 untreated foil: 450

C 168v C After heating l6 hrs. at 125C, the resistivity decreased Y I to 474 oh'ms cm. at 25C. The voltage capabilities are EXAMPLE 10 25 f l z 5 g. of salicylic acid and 2.5 boric acid are mixed in 100ml. of dimethylacetamide and the mixture rev g 25 Al foil r il 4x5 acted with heatin to rovide a roduct showin the 415 p p g v max. 125C boiled 611; 455 followlng illustrative results. untreated 455 Ohm PH 1 EXAMPLE 13 before reacting at 25C 267 3.1 Boro-di-naphthalenediolate after 2 hrs. at 125C 21] 3.9 after 16 hrs. at l25C 186 5.1 12.66 g. boric ac1d and 0.8 g. 2,3-naphthalenediolare after 32 hrs. at 125C 181 9.1

mixed in 100 g. DMF and the mixture heated overnight at 125C. 6.33 g. glycol is added to form a solution of dimethylammonium pentaborate and dimethylammonium boro-di-naphthalenediolate in DMF glycol system. The following results are illustrative:

Resistivity after 1,000 hours at 125C 650 ohm cm.

Formation of aluminum foil in this electrolyte shows the following: 4

Max. formation voltage at:

25C 174v 125C 135v Life test of 7 units, 200V at l25C, Averages:

F RC EXAMPLE 11 0 hr. 28.92 39 7.6 Borodwatecholate 1000 hrs. 28.88 40 5.6

4.24 g. of pyrocatechol and 2.65 g. of boric acid were mixed in l00 g. of DMF and the mixture reacted with I heating to provide a product with the following illustra- The heatmg Process provlded a conversion Saltstive results It is an advantage that the heating does not impair the ability of the electrolyte to form untreated foil at 25C but rather improves it slightly.

Ohm PH It has also been found that the salts formed according before reacting at 2240 23 to this invention may be prepared independently of the after 2 hrs. at.l25C 261 4.7 other electrolyte components, such as glycol. The folafter 16 hrs. at C I67 6.7 i after 34 hrs. at 125C 64 (L7 lowing example demonstrates the preparation not in an 0 electrolyte system. Formation of aluminum foil in this electrolyte shows EXAMPLE 14 th f ll i 4 g. of dimethylammonium pentaborate were prepared by heating boric acid in DMF for 16 hours and isolating the salt. 0.33 g. of dimethylammonium borogflg 152V dicatecholate were prepared by heating a mixture of 125C V boric acid and catechol in DMF overnight and isolating the salt.

An electrolyte system was formulated as follows:

DMF 79 g. Glycol g. Dimethylammonium 4 g.

pentaborate Dimethylammonium boro-di-catecholate The resultant electrolyte was unaffected by heating at 125C and no new salts were formed. The resistivity both before and after heating was 525 ohms-cm. The formation capabilities were unchanged on heating. Both before and after heating 16 hours at 125C the results were as follows:

EXAMPLE l5 Formation of aluminum foil in the electrolyte of Example 14 shows the following:

V max. 25C Al foil preboiled: 485 untreated foil: 465

V max. l25C boiled foil: 472 untreated foil: 468

lyte system within the electrolytic capacitor. Further, this invention provides a capacitor electrolyte containing an acid and a substituted amide which maintains a high voltage rating over a period of service. Specifically the novel electrolyte system exhibits no significant change in resistivity over a period of time.

As evidenced by the foregoing data, the formulations presented here result in capacitors having improved qualities and properties over previously known devices, particularly in the stability of high voltage ratings.

What is claimed is:

1. An electrical capacitor comprising a capacitor housing; a capacitance section within the housing, said section having at least a pair of spaced electrodes one of which is coated with a dielectric film; and an electrolyte in contact with said electrodes; said electrolyte consisting essentially of a solvent and a solute prepared from a liquid mixture of a substituted amide and an acid selected from the group consisting of l) phosphotungstic acid, (2) a combination of salicylic acid and boric acid, (3) a combination of pyrocatechol and boric acid, and (4) a combination of 2,3- naphthalenediol and boric acid, said mixture being reacted at about 125C for at least 2 hours and said electrolyte having a low resistivity.

2; A capacitor as claimed in claim 1 having a resistivity of from about to 600 ohm-cm. at C.

3. A capacitor as claimed in claim 1 wherein the substituted amide is dimethylformamide.

4. A capacitor as claimed in claim 1 wherein the substituted amide is dimethylacetamide.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 1 3,9 32 DATED Se tember 30, 1975 mvmrords) Franz S. Dunkl et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 68, correct the spelling of showing Column 2, line 37, the reaction should read ,N02 0 CH3 N0 r|- T o n -OH H-C-N\ co +0 4 (CH3)2NH2 N0 CH3 N02 Column 3, line 38, "91.9Z)" should read (91.9%)

Signcd and Scaled this A ttes t.

RUTH C. MASON C. MARSHALL DANN Arresting Officer (ommisxirmer nj'Patenls and Trademarks UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 1 3,909,682

DATED September 30, 1975 INVENTOR(S) Franz S. Dunkl et a1 It is certified that enor appears in the ab0veidentified patent and that said Letters Patent are hereby corrected as shown below: 9

Column 1, line'68, correct the spelling of showing Column 2, line 37, the reaction should read ,NO 3 CH T N0 O2N0H H-C-N\ C0 0 (CH NH N0 CH3 N0 Column 3, line 38, "91.92)" should read (91.9%)

Signcd and Scaled this second Day Of March 1976 [SEAL] Attest:

. RUTH MASON c. MARSHALL DANN Arresting Officer Commissioner oj'larents and Trademarks 

1. AN ELECTRICAL CAPACITOR COMPRISING A CAPACITOR HOUSING, A CAPACITANCE SECTION WITHIN THE HOUSING, SAID SECTION HAVING AT LEAST A PAIR OF SPACED ELECTRODES ON OF WHICH IS COATED WITH A DIELECTRIC FILM, AND AN ELECTROLYTE IN CONTACT WITH SAID ELECTRODES, SAID ELECTROLYTE CONSISTING ESSENTIALLY OF A SOLVENT AND A SOLUTE PREPARED FROM A LIQUID MIXTURE OFF A SUBSTITUTED AMIDE AND AN ACID SELECTED FROM THE GROUP CONSISTING OF (1) PHOSPHOTUNGSTIC ACID, (2) A COMBINATION OF SALICYLIC ACID AND BORIC ACID, (3) A COMBINATION OF PYROCATECHOL AND BORIC ACID, AND (4) A COMBINATION OF 2,3-NAPHTHALENEDIOL AND BORIC ACID, SAID MIXTURE BEING REACTED AT ABOUT 125*C FO AT LEAST 2 HOURS AND SAID ELECTROLYTE HAVING A LOW RESISTIVITY.
 2. A capacitor as claimed in claim 1 having a resistivity of from about 100 to 600 ohm-cm. at 125*C.
 3. A capacitor as claimed in claim 1 wherein the substituted amide is dimethylformamide.
 4. A capacitor as claimed in claim 1 wherein the substituted amide is dimethylacetamide. 